1. Field of the Invention
This invention relates to catalysts for the oxidation of sulfur dioxide. More particularly it relates to a process for preparing such catalysts.
Sulfuric acid is produced on a commercial scale by the vanadium-catalyzed oxidation of sulfur dioxide to sulfur trioxide in the gaseous phase followed by absorption of the sulfur trioxide in an aqueous solution.
It is advantageous to disperse the vanadium active element on a microporous carrier or support. The support has a variety of beneficial functions including a high surface area to increase reaction rate, high porosity to maximize diffusion, physical integrity to minimize dusting, thermal resistance so that the vanadium composition may be molten without disintegration or fusion of the catalyst particles, molded shaped form to minimize pressure drop, and chemical inertness. Until the instant invention a process for preparing catalysts with both high activity and outstanding durability in the form of shaped articles larger than cylinders of one cm diameter suitable for commercial production has not been disclosed.
2. Description of the Prior Art
The addition of many other metals to a potassium/vanadium oxidation catalyst on kieselguhr was disclosed by Simecek et al, J. Catalysis 14, 287-292 (1969); 18, 83-89 (1970) with the intent of lowering the temperature of maximum activity for their catalysts. These workers employed ammonium as the solubilizing cation, bisulfate as the solubilizing anion, sodium oxide as a source of sodium and operated in an unknown pH range. There is no disclosure that these workers intended or achieved a more durable catalyst by addition of sodium.
Alkali metal salts mixed with iron and manganese were added to vanadium oxidation catalysts by Topsoe and Nielsen, Trans. Dan. Acad. Tech. Sci 1, 3-23 (1947). These workers intended to increase the viscosity of the melted catalyst surface. They did not measure durability, they mixed additives, employed sulfate and often pyrosulfate as solubilizing anions, maintained highly alkaline conditions, and found less not more catalytic activity upon partial substitution of potassium by sodium.
In Japanese patent application No. 76-139, 586 (1976); C.A. 86, 111639k (1977) (cf. British Specification No. 1,520,336 (1978)) Hara et al. disclose the admixture of potassium and sodium sulfates expressed as oxides on a vanadium catalyst of pelletized diatomaceous earth. Only conversion not durability data are given. In their impregnation method potassium sulfate may form an interfering precipitate, since sulfuric acid not sulfite is employed as the neutralizing agent. No mention is made of the presence of crystalline domains. The sodium ions adventitiously present were derived from commercial impurities and were considered undesirable.
In U.S. Pat. No. 3,448,061 granted to Mika on June 3, 1969 the use of alkali metal compounds as promotors in catalysts containing vanadium is disclosed. The use of alkali metal compounds is restricted to the neutral range of pH from 6.5 to 9, with pH 7.5 to 8.5 preferred.
3. Objects of the Invention
An object of the present invention is to provide a hardened vanadium catalyst which can be utilized in the form of shaped articles as durable, useful catalysts in the "contact" process for the oxidation of sulfur dioxide. Another object of the invention is to provide catalysts with a high level of activity over a wide range of temperatures for the "contact" oxidation process. Other objects of the invention will be apparent to those skilled in the art from the description and examples below.